This invention relates to vapor axial deposition (VAD) processes for manufacturing optical fiber preforms.
A variety of methods are known for making optical fiber preforms in the manufacture of optical fiber including, for example, Modified Chemical Vapor Deposition (MCVD), Sol-Gel, and Vapor Axial Deposition (VAD). In the VAD method soot preforms are prepared by reacting glass precursors in an oxyhydrogen flame to produce silica particles. The silica particles are deposited on a starting rod. The starting rod is slowly pulled upward while it is rotated, and the silica particles are deposited axially on the rod as it is pulled. Very large, and long, soot preforms can be continuously fabricated. Typically the soot for the core is produced by a core torch and the soot for the cladding by a cladding torch. In this way, the composition of the glass can be varied from the center portion of the prefortm to the outside portion. Variation in glass composition is required for providing the refractive index difference necessary to produce light guiding in the optical fiber. After the soot is deposited, the preform is heated to consolidate the silica particles into a solid transparent glass body. Optical fiber is manufactured by drawing fiber from the consolidated preform using a conventional fiber drawing apparatus.
It has been recognized that the main functional part of an optical fiber is the core and the inner cladding. This part of the fiber carries 99+% of the optical energy. However, it typically consists of but 5% of the mass of the optical fiber. Accordingly, state of the art manufacture often makes use of an inner portion constituting core and inner clad region fabricated by soot deposition using MCVD or VAD, then overcladding the core rod with a material of less demanding properties. Consequently, the overcladdingxe2x80x94the bulk of the preformxe2x80x94may be produced by less costly processing. Overcladding may entail direct deposition on the core rod, or may result from using a separate xe2x80x9covercladding tubexe2x80x9d. Such overcladding tubes have been produced from soot or fused quartz.
A persistent problem with VAD methods is unwanted spatial variations in the composition on the soot preform as the preform is axially pulled. Variations in glass composition radially in the preform (x-y plane, with the x-y plane being the optical fiber cross section) are necessary to produce the light guide as just described. However, composition variations longitudinal of the preform (z-direction) as it is pulled are not desired. These variations lead to changes in the properties of the optical fiber along the length of the fiber.
Typically x-y plane variations are minimized by rotating the preform as it is pulled. However, there is no corresponding expedient for avoiding variations in the z-direction. These are time dependent variations and are produced by dynamic changes in the process conditions. Mostly, they result from changes in temperature, flow rates, and composition of the glass precursors. Of these, controlling the temperature is the most difficult. We have recognized that controlling the temperature of the process at the position where the core reaction occurs, i.e. the tip of the preform, is the important step in controlling the overall process.
European Patent No. EP 0 698 581 A2 describes an attempt to control the tip temperature. In the method described in that patent the temperature of the tip of the soot preform is monitored, and fed back to a flow control. The reaction temperature is dynamically controlled by changing the amount of Ar gas in the combustible mixture. The Ar gas acts as a diluent, so that increasing the Ar flow reduces the reaction temperature, and vice versa. We have found that this has an effect, but the effect is too small to eliminate the temperature fluctuations characteristic of many VAD processes.
We have developed a method for dynamically controlling the reaction temperature at the tip of a soot preform by controlling the flow of hydrogen gas to the core torch. This method allows significantly improved control of the temperature over the range necessary to eliminate typical variations.